1. Field of the Invention
The invention relates to a process for the production of boards of wood-based material, and to an installation for carrying out the process.
2. Description of Related Art
Application of the high-frequency technique as a means for preheating chip or fiber stock for the purpose of reducing the pressing factor during the subsequently initiated pressing operation to increase production output is generally known from the patent literature and general practical applications. It is known from U.S. Pat. No. 4,018,642 to use microwave as thermal energy for plywood, particle boards, chipboards and waffle boards, traveling waves being directed onto the pressed stock by means of so-called wave rectifiers at a frequency in the range from 100 to 10,000 Mhz. This U.S. Pat. 4,018,642 deals essentially with the preheating and curing of alkaline phenolic resins and similar glue compositions. Similar to the preheating mechanisms by microwave in food and kitchen technology, the energy utilization is .ltoreq.50%, which, for example in the application for preheating loosely laid mats of pressed stock, has the consequence that very long high-energy preheating zones are required, to be precise, depending on the height of the loosely laid stock, preheating zones in such high-frequency fields of 10 m to 20 m in length are used. Since the raising of the temperature gradient, for example from room temperature, takes place only very slowly and also brings about a drying out of the glues over the long preheating zone, and also with regard to premature gelling of the glues, a raising of the temperature is only possibly up to about 60.degree. Celsius. The essentially application-specific problems and risks of high-frequency heating are non-uniform thickness of the mat of pressed stock, control difficulties of the high-frequency energy to be supplied and disruptive discharges occurring. To cope with these difficulties, specific compaction measures between the microwave stations are described in DE 21 13 763 B2.
The problem of the previous, conventional preheating of wood products by means of high-frequency of microwave energy is that the period of heating up takes too long, or the heating zone for achieving a pressed stock temperature of 80 to 85.degree. Celsius, which is the aim, is too long. With a shallow energy-introduction and energy-absorption angle .alpha., a further temperature increase in the core of the pressed stock must be stopped at just 60.degree. Celsius, because with the binders (glues) used so far the chemical reaction for curing is initiated from only 60.degree. Celsius, that is to say the glue/binder contained in the mat of pressed stock begins to cure before the pressed stock reaches the run-in of the heated press. Responsible for this is also the water moisture contained in the pressed stock, which turns into steam as heating up is continued and consequently accelerates the setting process. This means that the board of wood-based material produced over the pressing zone or pressing time must have gained in strength, at least from the 100.degree. Celsius vapor point in the center of the mat of pressed stock up until it leaves the pressing zone or the press, corresponding to the setting zone X or the setting time X, to such an extent that when it leaves the press or the pressing zone the board has a transverse tensile strength which counteracts the residual vapor pressure still acting in the board. If the setting zone X or the setting time X is set too low, eruptions which destroy the surface and the product are produced as a result on leaving the pressing zone. Eruptions in the board of wood-based material can also be prevented by choosing the setting zone X to be appropriately long or the setting time X to be approximately great. With regard to production dependability, this means that either the pressing cycle in a platen press or the steel belt speed of the continuously operating press must be reduced or a correspondingly longer press must be constructed.
On the other hand, it is known from EP 0 383 572 A1 that, with the increased supply of energy caused by the steam shock, much shorter pressing factors or extremely short press lengths of a continuously operating press can be used. However, this is with the great disadvantage that an inadequate counter-pressure builds up in the core of the board by means of steam, heat and pressure at the beginning of the pressing zone on account of the increased plasticity of the wood-based material in the board, with the result that the board produced has a completely inadequate transverse tensile strength in practical applications. For this reason, this process never reach industrial application.